Passive multipath target range and depth estimation using a variable depth sonar

ABSTRACT

A variable depth sonar is disclosed which is adapted to provide passive multipath target ranging and depth estimates. The sonar employs a novel estimation procedure which is adapted to sonars with relatively small sensors and limited computing power. A novel feature of the invention is the introduction of a change of depth of the sonar receiver, mounted on a mobile platform, into the estimation process. Measurements at the two different depths are combined to provide target range and depth. 
     Other features and improvements are disclosed.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention is related to the field of passive sonar, and moreparticularly to passive sonar systems adapted to provide target rangeand depth information.

2. Description of the Prior Art

Passive sonar systems, unlike active systems that can send out energyand await an echo return, must obtain range estimates based entirely ontarget radiated energy. Most passive sonar systems do not provideestimates of the range to targets that have been detected. Those systemsthat do provide estimates obtain time difference data either betweenlargely separated array subsections or between complicated multipathsignal structures. Thus, unless the sonar has a large aperture array oran extremely large computer to correlate multipath returns with allpossible multipath structures, the estimate of range is not obtained.

It is, therefore, an object of the present system to provide a passivesonar system adapted to provide target and range information.

A further object of the invention is to provide a passive sonar systememploying a relatively small sensor with limited computing power whichis adapted to provide target range and depth information.

Yet another object of the present invention is to provide a simple rangeand depth estimation technique for mobile sonar systems.

SUMMARY OF THE INVENTION

A variable depth mobile sonar is disclosed which is adapted to estimatetarget range and depth. It is assumed that acoustic energy is propagatedfrom the target to the sonar via a direct path and a surface reflectedpath. The invention utilizes multipath time differences at differentdepths to provide both range and depth estimates of the target. Passivemeasurements are taken at two different depths. Based upon the knownsonar receiver depths, the velocity of the sonar receiver, and the pathlength differences estimated by utilizing the autocorrelation functionof the received waveform, estimates of the target depth and range arecalculated using novel algorithms.

Other features and advantages are disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The various objects, features and advantages of the disclosed inventionwill be readily appreciated by persons skilled in the art from thefollowing detailed disclosure when read in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a diagram illustrating the relative underwater locations ofthe sonar platform and the target, and defining several pertinentparameters.

FIG. 2 is a modification of the diagram of FIG. 1, illustrating thegeometric approximations used to derive the estimation algorithms.

FIG. 3 is a block diagram of the preferred embodiment of the invention.

FIG. 4 is a flow chart illustrating the operations of the preferredembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention comprises a novel variable depth mobile passivesonar system. The following description of the invention is provided toenable any person skilled in the art to make and use the inventions.Various modifications to the disclosed embodiment will be readilyapparent to those skilled in the art, and the general principles definedherein may be applied to other embodiments. Thus, the present inventionis not intended to be limited to the embodiment shown, but is to beaccorded the widest scope consistent with the principles and novelfeatures of the invention.

The invention is adapted for use on a mobile platform such as by way ofexample only, a variable depth towed sonar, submarine or torpedo. Theproperties exploited in the preferred embodiment are that (1) theplatform can compare estimates or measurements made at different knowndepths, and (2) for a submerged target, the acoustic energy propagationis primarily via a direct and a surface reflected path.

For the purposes of the preferred embodiment, the direct propagationpath is assumed to be straight-line, and the reflection form the surfacefor the reflected path is assumed to be perfect. The source is assumedto be stationary and a strong emitter of acoustic energy, and to be muchfarther from the platform than its depth. The platform is assumed to bemoving horizontally, i.e., at a constant depth and at a constant speed,and its depth is assumed known without error.

FIG. 1 depicts these assumed conditions, where

d₁ =platform depth

d₂ =target depth

v=platform speed

c=sound speed in the medium

R=horizontal range to target

L₁ =length of direct propagation path

L₂ =length of surface reflected path

s(t)=source random radiated waveform (from target)

y(t)=waveform received at the platform

If the signal s(t) is very strong, the noise may be neglected at theplatform receiver. The received waveform y(t) is the sum of the signalsdelayed from the two propagation paths, ##EQU1##

The autocorrelation function for the signal s(t) may be denoted by R_(s)(τ), and the received signal y(t) autocorrelation function may bedenoted by R_(y) (τ). The difference Δ in the propagation times betweenthe direct signal path L₁ and the surface reflected signal path is (L₂-L₁)/c. Thus, the relationship between the two autocorrelation functionsfor the received signal y(t) may be expressed by Equation 2.

    R.sub.y (τ)=2 R.sub.s (τ)+R.sub.s (τ+Δ)+R.sub.s (τ-Δ)                                           (2)

The received autocorrelation function R_(y) (τ) contains the signalcorrelation function R_(s) (τ) shifted by Δ seconds from the origin,i.e., shifted by the time difference for propagation over the two paths.For strong signals, the peaks in the autocorrelation function are easilydetected, and thus a good estimate of the time difference Δ is obtainedfrom the difference between peaks in the received autocorrelationfunction R_(y) (τ).

The assumption that the range of the target is much greater than itsdepth implies that L₁ ² >>d₁ d₂ and that L₁ ≈R. It is straight-forwardto show that ##EQU2##

Reference to FIG. 2 illustrates the assumed geometric relationships bywhich the approximations of Equation 3 are reached. The triangle whoselongest side is of length L₂ has second and third sides of length R and(d₁ +d₂). Since it is assumed that the target's range is much greaterthan its depth, this triangle is assumed to be a right triangle. Hencethe relationship of Equation 4 is obtained.

    L.sub.2.sup.2 ≈R.sup.2 +(d.sub.1 +d.sub.2).sup.2   (4)

Similarly, the right triangle whose hypotenuse is L₁ has sides of lengthR and (d₂ -d₁), from which the relationship of Equation 5 is obtained.

    L.sub.1.sup.2 =R.sup.2 +(d.sub.2 -d.sub.1).sup.2           (5)

The relationship of Equation 3 follows readily from the relationship ofEquation 4 and 5, and the use of the approximation that ##EQU3## wherea<<1.

The relationship of Equation 3, with two measurements of the timedifference Δ, at two different times and depths, is used to solve forthe unknowns d₂ and R that are the parameters to be estimated.

At time t₁, the platform depth d₁ (t₁), its velocity v, and the timedifference Δ(t₁) is measured from the correlation function. Thus, attime t₁, ##EQU4##

The platform then changes depths. At time t₂, the new platform depth d₁(t₂) and time difference Δ(t₂) are measured. Since it is assumed thatthe target was not moving at all, and the platform is heading directlyto the target (which is reasonable for high signal-to-noise ratio), therange to the target is reduced by the velocity times the time intervalbetween measurements. ##EQU5##

From the relationships for the measurements at the two times t₁, t₂, asolution is available for R and d₂ : ##EQU6##

Thus, the preferred embodiment provides approximate range and targetdepth.

The operation of the preferred embodiment is further illustrated in FIG.3. Sonar receiver 200 is coupled to an acoustic energy transducer, andprovides an output receiver waveform y(t). The output of receiver 200 iscoupled to correlator 210, which comprises a parallel bank ofautocorrelators. Each autocorrelator comprises a delay element 212 fordelaying the signal y(t) a predetermined time, a mixer 214 for mixingthe signal y(t) and the delayed counterpart signal to provide a productsignal y(t)y(t+delay), and an integrator or smoother circuit 216 adaptedto provide a good estimate of the product. The integrator 216 integratesthe product signal over a time T, whose value is selected to provide agood estimate on the peak.

Each of the autocorrelators is adapted to introduce a time delay whichis an integral multiple of a constant delay Δ. Thus, for the bank of Nautocorrelators illustrated in FIG. 3, the first autocorrelatorintroduces a delay of Δ seconds, the second autocorrelator introduces adelay of 2 Δ seconds, and the Nth autocorrelator introduces a delay of NΔ seconds. The magnitude of Δ determines the resolution of correlator210.

The outputs of each of the autocorrelators is provided to centralprocessor 240. Clock 230, platform depth indicator 250 and platformvelocity indicator 260 are also coupled to processor 240, to providesignals representative of time, platform depth and platform velocity.

The processsor 240 is adapted to read the values of each autocorrelatorand to identify, for a given time t_(i), the autocorrelator whose outputis the largest value. The delay introduced by the selectedautocorrelator is determined to be the delay estimate Δ(t_(i)) for thegiven time t_(i).

The actual implementation of the processor and correlator is a manner ofchoice, as each of the components comprising the correlator, as well asprocessors, are well known to those skilled in the art. For example, apreferred implementation of the correlator comprises a shift register,with each autocorrelator delayed term being taken off a tapped delayline.

The operation of the invention is further illustrated by the flow chartof FIG. 4. At step 300 and the commencement of operation, the platformis at initial depth d₁ ; the processor determines this depth and storesit for subsequent use in the estimation algorithms. At step 305, theprocessor reads the value of each autocorrelator at time t₁. At step310, the processor selects the largest autocorrelator output todetermine the appropriate time delay estimate Δ(t₁), and stores thisestimate for subsequent use.

At step 315, the platform is moved to depth d₂. At time t₂, during step320, the processor reads the value of each autocorrelator output. Atstep 325, the correlator having the largest output value is selected todetermine the appropriate delay estimate at time t₂, Δ(t₂).

At step 330, the platform speed is determined. After this step, theprocessor has the necessary information to carry out the estimationalgorithms of Equation 8 and 9.

The invention may also be viewed as a method for estimating the targetrange and depth. The method comprises the steps of: (a) positioning apassive sonar receiver at a first depth; (b) determining an estimate ofthe difference in acoustic energy propagation time for a directpropagation path and a surface reflected path at time t₁ ; (c)positioning the sonar receiver at a second depth d₂ ; (d) determining anestimate of such difference at time t₂ ; (e) determining the sonarreceiver's velocity, and (f) calculating estimates of the target's rangeand depth utilizing the estimation algorithms of Equations 8 and 9.

It is understood that the above-described embodiment is merelyillustrative of the many possible specific embodiments which canrepresent applications of the present invention. Numerous and variedother arrangements can be readily devised in accordance with theseprinciples by those skilled in the art without departing from the spiritand scope of the invention.

What is claimed is:
 1. A passive sonar system adapted to provideestimates of a target location, comprising:(a) receiver means adapted toreceive acoustic energy from such target and provide receiver signal;(b) receiver positioning means adapted to move said receiver means froma first depth to a second depth; and (c) processing means coupled tosaid receiver means and adapted to process the receiver signalsgenerated by said receiver at said first depth and at said second depth,and provide sonar signals indicative of the target's location.
 2. Thesonar system of claim 1 wherein said sonar signals are indicative ofestimates of the target's range from the receiver and its depth from thesurface.
 3. The sonar system of claim 2 wherein said processing meanscomprises means for approximating the acoustic propagation timedifferential between two possible acoustic propagation paths at saidfirst depth and at said second depth.
 4. The sonar system of claim 3wherein said means for approximating the time differential comprisescorrelator means adapted to estimate a delay time value which provides arelative maximum in the autocorrelation of the receiver signals.
 5. Thesonar system of claim 4 wherein said correlator means includes aplurality of autocorrelator means, each respectively adapted todetermine the autocorrelation of the receiver signal as a function ofpredetermined time delay values.
 6. The sonar system of claim 3 whereinsaid processor means comprises computer means for determining anestimate R of the range from the algorithm ##EQU7## where v is thevelocity of the receiver means, t₁ is the time at which the receivermeasurement at the first depth d₁ occurs, t₂ is the time at which thereceiver measurements at the second depth d₂ occurs, Δ(t₁) is themeasured time differential at said first depth d₁, and Δ(t₂) is themeasured time differential at said second depth d₂.
 7. The sonar systemof claim 6 wherein said processor means further comprises computer meansfor carrying out the following algorithm for determining an estimate Dof the target depth: ##EQU8## where c is the speed of sound in themedium.
 8. A method of estimating target depth and range using a dualdepth sonar, comprising the steps of:(a) positioning a sonar receiver ata first depth d₁ ; (b) determining a first estimate Δ(t₁) of thedifference in the acoustic energy propagation times for a directpropagation path and a surface reflected path at a first time t₁ and atsaid first depth; (c) positioning the sonar receiver at a second depthd₂ ; (d) determining a second estimate Δ(t₂) of said difference inpropagation times at a second time t₂ and at said second depth; (e)determining an estimate of the sonar receiver's velocity; and (f)calculating an estimate of the target's range from the sonar receiver,based upon said first and second estimates of the difference inpropagation times, said estimate of sonar receiver velocity, and thetime interval from said first time t₁ said second time t₂.
 9. The methodof claim 8 wherein said step of calculating estimates of the target'srange comprises providing a processor means adapted to calculate theestimate R of the range from the following alogirthm: ##EQU9## where vis the velocity of the receiver means, t₁ is the time at which thereceiver measurement at the first depth d₁ occurs, t₂ is the time atwhich the receiver measurements at the second depth d₂ occurs, Δ(t₁) isthe measured time differential at said first depth d₁, and Δ(t₂) is themeasured time differential at said second depth d₂.
 10. The method ofclaim 8 further comprising the step of:calculating an estimate of thetarget's depth D, based upon said estimate of the target's range R, thevelocity c of acoustic energy propagation in the medium, and saidreceiver first depth d₁.
 11. The method of claim 10 wherein said step ofcalculating the target depth comprises providing a processor adapted tocalculate an estimate D of the target's depth using the followingalgorithm: ##EQU10## where c is the speed of sound in the medium.